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•The particle disintegration of cooked milled rice plays key role in starch digestibility.•Low GI rice varieties differentiated by the reaction rate of initial fast phase.•Less ...small/intermediate disintegrated cooked grain particles give slower starch digestibility.•Particle disintegration parameter Q-300 can predict GI of cooked rice grain digestibility.
This study investigated the particle disintegration of cooked milled rice during in vitro digestion to identify its potential for rapidly predicting glycaemic index (GI). Milled grains and flour of rice with varying GI were cooked, stirred and subjected to digestion followed by kinetics analyses. Despite variations in physicochemical parameters (typically amylose content), flours showed a single-phase-digestion rate (k, ∼0.12 min−1) which did not vary significantly between varieties. In contrast, intact grains were disintegrated into small/intermediate (d < 30 μm, 30 < d < 300 μm) and large (d > 300 μm) particles. The small/intermediate particles comprising 50–70 % starch were initially-digested (0−20 min) at a fast k-f (∼0.05−0.10 min−1), which enabled to differentiate rice digestibility; whereas the large was latter-digested (20–180 min) at a slow k–s (∼0.04 min−1). The sum-ratio of disintegrated-particle 0−300 μm (Q-300) correlated positively with clinical GI values, allowing for a digestibility prediction of intact milled rice grain.
The current incidence of obesity and type 2 diabetes is at global epidemic levels. To mitigate their impact, there is a need to develop starch-containing foods that give rise to a low and stable ...postprandial blood glucose response by increasing the proportion of slowly-digestible and indigestible carbohydrate content. Rice is an ideal target food for such dietary intervention because it is a staple food for over half the world's population.
The starch digestion of cooked white rice grains is usually complete or near complete upon consumption, but the rate of digestion is influenced by intrinsic food properties and extrinsic influences. This review provides an overview of the complex interplay between the starch granule and its interaction with non-starch components of the rice grain (intrinsic characteristics) as well as the effects of processing (extrinsic factors) on starch digestibility.
The intrinsic properties of white rice grains play a significant role in starch digestibility which can be further enhanced after processing, especially by gelatinisation and retrogradation. Post-harvest storage conditions of rice were found to influence starch digestibility but this effect was temperature-dependent. Limited studies investigated starch-lipid and starch-protein interactions in rice, but changes to substrate accessibility have been implicated. Improving our understanding of the effects of processing on starch digestibility can provide an effective tool for food manufacturers to regulate starch digestibility of existing rice varieties.
•Obesity and type 2 diabetes are at worldwide epidemic proportions.•Rice is a staple food crop and an ideal target food for dietary intervention.•Starch digestibility is influenced by intrinsic rice characteristics.•Processing may be a useful way to manipulate starch digestion properties.
To ensure rice food security, the target outputs of future rice breeding programmes should focus on developing climate-resilient rice varieties with emphasis on increased head rice yield coupled with ...superior grain quality. This challenge is made greater by a world that is increasingly becoming warmer. Such environmental changes dramatically impact head rice and milling yield as well as increasing chalkiness because of impairment in starch accumulation and other storage biosynthetic pathways in the grain. This review highlights the knowledge gained through gene discovery via quantitative trait locus (QTL) cloning and structural–functional genomic strategies to reduce chalk, increase head rice yield, and develop stable lines with optimum grain quality in challenging environments. The newly discovered genes and the knowledge gained on the influence of specific alleles related to stability of grain quality attributes provide a robust platform for marker-assisted selection in breeding to design heat-tolerant rice varieties with superior grain quality. Using the chalkiness trait in rice as a case study, we demonstrate here that the emerging field of systems genetics can help fast-track the identification of novel alleles and gene targets that can be pyramided for the development of environmentally robust rice varieties that possess improved grain quality.
•Starch granule digestion studied for rice lines differing in crystalline polymorph.•Both A- and B-type crystalline granules have surface pores.•Patterns of hydrolysis similar but B-type granules ...more enzyme resistant.•Suggests that amylopectin branch lengths are the major underlying factor.
The underlying mechanism of amylolysis of rice starch granules was investigated using isolated starch granules from wild-type, as well as SBEIIb mutant and down-regulated lines. Fused granule agglomerates isolated from mutant and transgenic lines were hydrolysed at similar rates by amylases, and had similar crystalline patterns and thermal properties as individual granules. Surface pores, a feature previously only reported for A-polymorphic starch granules, were also observed in B- and C-polymorphic rice starch granules. Although the microscopic patterns of hydrolysis among granules with different crystalline polymorphs were qualitatively similar, the extent and the rate of amylolysis were different, suggesting that B-type crystalline polymorphs are intrinsically more resistant to enzymatic hydrolysis than A-type in rice starch granules. It is proposed that the slightly longer branch lengths of amylopectin which leads to the formation of more stable B-type double helical structures compared to their A-type counterparts is the major parameter, with other factors such as granule size, surface pores and interior channels having secondary roles, in determining the rate of enzymatic hydrolysis of rice starch granules.
A global rise of diet-related noncommunicable diseases calls for a focus on diet-based nutritional intervention across the entire socioeconomic consumer spectrum. We review recent reports in the area ...of healthier rice aimed at developing rice grains with improved dietary fiber compositions (increased amounts of nonstarch polysaccharides and resistant starch), and less digestible starch (higher amylose and phospholipid complex in the endosperm) resulting in reduced glycemic impact upon grain consumption. We furthermore elaborate on the interconnections of elevated amounts of protein and a balanced composition of essential amino acids. The importance of a nutritious aleurone layer and its role in lipid storage and micronutrient composition is discussed briefly in the context of brown rice benefits. We identify gene targets for precision breeding that will facilitate the production of rice grains and rice-based products to mitigate the impact of nutrition-related preventable diseases.
Rice grain quality encompasses complex interrelated traits that cover biochemical composition, cooking, eating, nutritional, and sensory properties. Because rice endosperm is composed mainly of ...starch, rice grain quality is traditionally defined by characterizing starch structure and composition, which is then subsequently correlated with functional properties of the grain. The current proxy tests routinely used to describe rice grain quality preferences are rather limited to the estimation of apparent amylose content, gelatinization temperature, and gel consistency. Additional tests that characterize starch property, viscoelasticity, grain texture, and aroma are also employed in more advanced laboratories. However, these tests are not routinely applied in breeding programs to distinguish cooking quality classes to reflect evolving consumer preference and market demand. As consumer preferences in Asia and all over the world are diverse due to varied demographics and culture, defining uniform attributes to capture regional grain quality preferences becomes more challenging. Hence, novel and innovative proxy tests are needed to characterize rice grain quality to meet the demand for consumer preferences of commercially-released cultivars. In this chapter, the current methods employed in rice grain quality monitoring are succinctly reviewed. Future prospects for improvement are identified, introducing cutting edge technologies that can facilitate high-throughput screening of rice diversity panels and breeding lines. Aside from addressing the requirements for quality improvement in the traditional inbred rice breeding programs, we also tackled the need to enhance grain quality in the hybrid rice sector.
A deeper understanding of the regulation of starch biosynthesis in rice (Oryza sativa) endosperm is crucial in tailoring digestibility without sacrificing grain quality. In this study, significant ...association peaks on chromosomes 6 and 7 were identified through a genomewide association study (GWAS) of debranched starch structure from grains of a 320 indica rice diversity panel using genotyping data from the high-density rice array. A systems genetics approach that interrelates starch structure data from GWAS to functional pathways from a gene regulatory network identified known genes with high correlation to the proportion of amylose and amylopectin. An SNP in the promoter region of Granule Bound Starch Synthase I was identified along with seven other SNPs to form haplotypes that discriminate samples into different phenotypic ranges of amylose. A GWAS peak on chromosome 7 between LOC_Os07g11020 and LOC_Os07g11520 indexed by a nonsynonymous SNP mutation on exon 5 of a bHLH transcription factor was found to elevate the proportion of amylose at the expense of reduced short-chain amylopectin. Linking starch structure with starch digestibility by determining the kinetics of cooked grain amylolysis of selected haplotypes revealed strong association of starch structure with estimated digestibility kinetics. Combining all results from grain quality genomics, systems genetics, and digestibility phenotyping, we propose target haplotypes for fine-tuning starch structure in rice through marker-assisted breeding that can be used to alter the digestibility of rice grain, thus offering rice consumers a new diet-based intervention to mitigate the impact of nutrition-related noncommunicable diseases.
The characterization of gene expression is dependent on RNA quality. In germinating, developing and mature cereal seeds, the extraction of high-quality RNA is often hindered by high starch and sugar ...content. These compounds can reduce both the yield and the quality of the extracted total RNA. The deterioration in quantity and quality of total RNA can subsequently have a significant impact on the downstream transcriptomic analyses, which may not accurately reflect the spatial and/or temporal variation in the gene expression profile of the samples being tested. In this protocol, we describe an optimized method for extraction of total RNA with sufficient quantity and quality to be used for whole transcriptome analysis of cereal grains. The described method is suitable for several downstream applications used for transcriptomic profiling of developing, germinating, and mature cereal seeds. The method of transcriptome profiling using a microarray platform is shown. This method is specifically designed for gene expression profiling of cereals with described genome sequences. The detailed procedure from microarray handling to final quality control is described. This includes cDNA synthesis, cRNA labelling, microarray hybridization, slide scanning, feature extraction, and data quality validation. The data generated by this method can be used to characterize the transcriptome of cereals during germination, in various stages of grain development, or at different biotic or abiotic stress conditions. The results presented here exemplify high-quality transcriptome data amenable for downstream bioinformatics analyses, such as the determination of differentially expressed genes (DEGs), characterisation of gene regulatory networks, and conducting transcriptome-wide association study (TWAS).
Summary
Reliably generating rice varieties with low glycaemic index (GI) is an important nutritional intervention given the high rates of Type II diabetes incidences in Asia where rice is staple ...diet. We integrated a genome‐wide association study (GWAS) with a transcriptome‐wide association study (TWAS) to determine the genetic basis of the GI in rice. GWAS utilized 305 re‐sequenced diverse indica panel comprising ~2.4 million single nucleotide polymorphisms (SNPs) enriched in genic regions. A novel association signal was detected at a synonymous SNP in exon 2 of LOC_Os05g03600 for intermediate‐to‐high GI phenotypic variation. Another major hotspot region was predicted for contributing intermediate‐to‐high GI variation, involves 26 genes on chromosome 6 (GI6.1). These set of genes included GBSSI, two hydrolase genes, genes involved in signalling and chromatin modification. The TWAS and methylome sequencing data revealed cis‐acting functionally relevant genetic variants with differential methylation patterns in the hot spot GI6.1 region, narrowing the target to 13 genes. Conversely, the promoter region of GBSSI and its alternative splicing allele (G allele of Wxa) explained the intermediate‐to‐high GI variation. A SNP (C˃T) at exon‐10 was also highlighted in the preceding analyses to influence final viscosity (FV), which is independent of amylose content/GI. The low GI line with GC haplotype confirmed soft texture, while other two low GI lines with GT haplotype were characterized as hard and cohesive. The low GI lines were further confirmed through clinical in vivo studies. Gene regulatory network analysis highlighted the role of the non‐starch polysaccharide pathway in lowering GI.
Summary
Resistant starch (RS) is the portion of starch that escapes gastrointestinal digestion and acts as a substrate for fermentation of probiotic bacteria in the gut. Aside from enhancing gut ...health, RS contributes to a lower glycemic index. A genome‐wide association study coupled with targeted gene association studies was conducted utilizing a diverse panel of 281 resequenced Indica rice lines comprising of ~2.2 million single nucleotide polymorphisms. Low‐to‐intermediate RS phenotypic variations were identified in the rice diversity panel, resulting in novel associations of RS to several genes associated with amylopectin biosynthesis and degradation. Selected rice lines encoding superior alleles of SSIIa with medium RS and inferior alleles with low RS groups were subjected to detailed transcriptomic, metabolomic, non‐starch dietary fibre (DF), starch structural and textural attributes. The gene regulatory networks highlighted the importance of a protein phosphatase alongside multiple genes of starch metabolism. Metabolomics analyses resulted in the identification of several metabolite hubs (carboxylic acid, sugars and polyamines) in the medium RS group. Among DF, mannose and galactose from the water‐insoluble fraction were found to be highly associated with low and medium RS lines, respectively. Starch structural analyses revealed that a moderate increase in RS is also linked to an elevation of amylose 1 and amylose 2 fractions. Although rice lines with medium RS content negatively affected textural and viscosity properties in comparison to low RS, the textural property of medium RS lines was in the same acceptable range as IR64, a rice mega variety popular in Asia.
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